In Sails around that size range, the Gator is an excellent match to the ST104 in choppy, fast freemove type conditions and the 5.0 would fit well with your existing S-2 5.8m.
If you wanted to go more into the wave sail range, the Blade 5.3m is probably the best choice, it delivers a lot of locked drive (for a wavesail). The Renegade is fun and very durable sail, but with a very soft feel, which is not so matched to the freerace/SuperX character of the ST&#39;s.